The \(K^+\rightarrow \mu ^+\nu \nu \bar{\nu }\)  decay occurs within the SM at second order in the Fermi constant \(G_F\) , and the expected branching fraction at leading order in chiral perturbation theory, \({\cal B_{\rm SM}}=1.62\times 10^{-16}\)  [1]}, is experimentally out of reach. The strongest upper limit to date, \({\cal B} (K^+\rightarrow \mu ^+\nu \nu \bar{\nu })<2.4\times 10^{-6}\)  at 90% CL, has been established by the BNL-E949 experiment [2]}.
